1. Field of the Invention
The present invention relates to an improvement in a control system for the induction motor to which power is supplied through an inverter of pulse width modulation (PWM) type.
2. Description of the Prior Art
The use of a control system for the induction motor with a PWM inverter is spreading in the field of rail vehicles and the like.
A PWM inverter is one in which by changing the width of pulses periodically, the average value of an output voltage is produced as a sine waveform while being controlled by a modulated wave which varies the pulse widths, thereby producing a sine waveform output in accordance with the modulated wave. This inverter is provided with means for minimizing the high harmonics contained in the output voltage at the time of modulation.
In a modulator for producing such a pulse train, it is common practice to use a system for comparing a sine wave with a triangular wave. Specifically, a sine wave, and a triangular wave which is higher in frequency than the sine wave are applied to a comparator, from the output of which a modulated pulse train is produced. The average voltage of each pulse forms a sine wave of the same frequency as the applied sine waveform. If the inverter is driven by this modulated pulse train, the average value of the inverter output voltage takes the form of a sine wave voltage.
The output voltage of the inverter can be changed by changing the ratio of the peak values between the sine wave and the triangular wave applied to the comparator.
In this PWM inverter, with the increase in the frequency ratio N of the triangular wave to the sine wave used for modulation, the lower-order harmonics contained in the output voltage are reduced. In the case where an output of very low frequency is required for the speed control of an induction motor, therefore, the frequency ratio N is increased thereby to prevent any torque pulsation which otherwise might occur by the lower-order harmonics.
In the case of a large frequency ratio N, however, there is such a disadvantage that the continuously controllable range of the output voltage is narrowed. Specifically, when the pulse width is enlarged in order to increase the output voltage, the slit width corresponding to a period of time when a thyristor is turned off and then turned on again, is narrowed, and when it is narrowed to less than a predetermined width depending on the turn-off time of the thyristor, a commutation error occurs, thus making it impossible to increase the output voltage beyond a certain level. In order to avoid this disadvantage, it is desirable to switch the frequency of the triangular wave before the output voltage reaches the above-mentioned level. In the speed control of an induction motor with a PWM inverter, the output voltage is increased in proportion to the frequency and, however, within the range in which the output voltage is limited by the turn-off time of the thyristor, it is unnecessary to use a triangular wave of such a high frequency as at the time of starting from the viewpoint of torque pulsations since the frequency of the range is higher than that at the time of starting. Accordingly, if the frequency ratio N of the triangular wave to the sine wave is reduced, the slit width of the pulse train may be increased, thus making it possible to further increase the output voltage of the inverter.
In this connection, it is necessary to consider the case of N being 1, in which the induction motor is running at high speed. Under that condition, the triangular wave and the sine wave have the same frequency, and therefore no slit is included during the half cycle of the pulse train for controlling the inverter, so that the output voltage of the inverter becomes uncontrollable. Although this poses no problem in either cases where the induction motor is in a powering mode or in a regenerative mode. If the induction motor starts being powered or enters a regenerative mode while it is running by inertia at high speed, (in such a case where an electric rolling-stock is running by inertia at high speed), the maximum output voltage of the inverter is applied to the induction motor from the beginning.
Moreover, the induction motor has only a reactance of the windings before generation of an induced voltage, and therefore, if a maximum voltage is applied thereto while it is running by force of inertia, a surge current flows into the induction motor, thus often leading to a commutation failure of the inverter. That phenomenon has been confirmed and a commutation failure has been observed in the case of a test conducted by the inventers.